As the miniaturization technique has advanced in recent years, circuits have become more complicated, and the costs related to the development of semiconductor chips have become higher.
In a reconfigurable logic circuit, circuit logics can be changed after manufacture of the hardware; accordingly, by reconfiguring logic circuits, defects can be corrected and functions can be added.
As reconfigurable logic circuits based on today's semiconductor techniques, there are integrated circuits called field programmable gate arrays (FPGAs). An FPGA stores information in internal SRAMs (Static Random Access Memories), and according to the contents stored in the memories, the FPGA can control the logics and connections in the reconfigurable logic circuit. Since logics can be changed through software, circuit corrections can be made after manufacture of the hardware. Such FPGAs are rapidly spreading as the means to produce ever more complicated integrated circuits in a short time and at low costs.
The logic section of each FPGA is formed by circuits called look-up table circuits. A look-up table circuit stores logics in a memory, and according to the contents stored in the memory, the look-up table circuit performs outputting. A logic circuit including such look-up table circuits is a reconfigurable logic circuit that can support any logics, however, since it includes a large number of devices, it is difficult to achieve high integration.
Where look-up table circuits are manufactured by a semiconductor CMOS technique, SRAMs are used as the memories to store information. Therefore, the number of devices becomes large. Since a multiplexer used in each look-up table circuit also requires a large number of devices, the circuit size of each look-up table circuit becomes very large.
If look-up table circuits with small circuit areas are used, a highly-integrated FPGA can be realized. As the look-up table circuits with small circuit areas, look-up table circuits using spin MOSFETs have been suggested. With the use of look-up table circuits using spin MOSFETs, a highly-integrated FPGA can be realized. A spin MOSFET has a structure that combines a ferromagnetic material and a semiconductor, and accordingly, a memory function and a logic function can be embodied by one device. That is why the spin MOSFETs have attracted attention in recent years.
A look-up table circuit using spin MOSFETs reads the resistance values of the spin MOSFETs, and determines an output in accordance with the resistance values. To read a resistance value, the look-up table circuit reads the voltage applied to the spin MOSFET, compares the voltage with the resistance value of a reference resistance, and determines the magnitude of the resistance value. The look-up table circuit uses a comparator as the circuit to determine the magnitude of the resistance value; however, since the operation speed of a comparator is low, the circuit has a problem in that its operation speed is also low accordingly. The operation time of a look-up table circuit can be regarded as the period of time elapsing from a change in input to a change in output. If the operating time of each look-up table circuit is long, the operation time of the entire FPGA is long; therefore, there is a demand for high-speed look-up table circuits. Particularly, in a case where multi-input signals for image processing or the like are dealt with, high-speed operations of the logic unit including look-up table circuits greatly contributes to an increase in processing speed.